The present invention relates to a solid state imaging device which is used to optically scan and read an image of a manuscript mainly in an image forming apparatus such as a copying machine or the like, and an image read apparatus using the present solid state imaging device.
As an image read apparatus for use in the above-mentioned copying machine, there is known an image read apparatus of a type that the image of a manuscript placed on a platen glass is scanned by an optical scanner, an image optical signal which is a reflected light from the manuscript image generated when it is scanned by the optical scanner is image formed by a lens on a solid state imaging device such as a CCD or the like, and the reflected light is scanned and converted photoelectrically into an electric image signal by the solid state imaging device, before it is output from the present image read apparatus. In the present solid state imaging device, a plurality of photosensitive pixels are arranged in point order in a main scanning direction crossing at right angles to the scanning direction of the optical scanner; for example, if a copying machine in which the present solid state imaging device is used is of a type that it can deal with full colors, then there are arranged a plurality of photosensitive pixel lines, each line including more than one set of three photosensitive pixels (R: red, G: green, B: blue) which are respectively able to separate the three primary colors, that is, red, green, and blue.
In the copying machine of a full-color type, since the size of the photosensitive pixel per color is decreased down to a third or so when compared with a normal monochrome type of copying machine, the MTF (Modulation Transfer Function) thereof in the main scanning direction is increased up to a high frequency. As conventionally known, the thus increased MTF results in the deteriorated image quality such as a moire, folding distortion, coloring, a blur and the like. In view of this, in the Unexamined Japanese Patent Application Publication No. Sho. 59-181568, there is disclosed a technology in which a photosensitive pixel is formed in a parallelogram shape to thereby lower the MTF of the photosensitive pixel in the main scanning zero-degree direction thereof so as to reduce the deterioration of image quality. Also, in the Examined Japanese Utility Model Publication No. Hei. 5-5719, there is disclosed a solid state imaging device in which there are arranged a plurality of photosensitive pixels each having a diamond shape.
Also, in the image read apparatus, in order to prevent the above-mentioned image deterioration effectively, an optical lowpass filter (a diffraction grating) is interposed between a lens and a CCD. For example, in the Unexamined Japanese Patent Application Publication No. Hei. 7-66948, there is proposed a technology in which between a lens and a CCD there are arranged a plurality of lowpass filters each having a linear pattern in such a manner that their linear patterns are oriented in mutually different directions to thereby remove interference patterns, which are caused to occur when diffracted lights generated from the lowpass filters interfere with each other, so as to obtain an image of high quality. Also, in some cases, the lowpass filters are not interposed between the lens and CCD but they are interposed between the manuscript and lens. Further, as the lowpass filter, a crystal plate can also be used.
According to the technology disclosed in the above-mentioned Japanese Patent Application Publication No. Sho. 59-181568, like a copying machine, in an image read apparatus which scans an optical signal not only in a main scan direction but also in a direction crossing at right angles to the main scan direction, as shown in FIGS. 28a and 28b, the reading range thereof is moved in a sub-scan direction (in FIGS. 28a and 28b, in the vertical direction) and the value of the integration thereof provides the image data. FIG. 28A shows an integration area when a copy image is 100%, while FIG. 28B shows an integration area when a copy image is enlarged by 400%. In enlargement, the scan distances of two scanners 11 and 12 are reduced in inverse proportion to the enlargement ratios, with the result that the scan distance in the sub-scan direction is shorter than when a copy image is 100%. In this case, the integration area provides a narrow parallelogram and, therefore, as described above, the MTF in the main scan zero-degree direction can be reduced, whereas the MTF in a C-C' direction crossing at right angles to an oblique side A-A' direction still remains high. Especially, in a copying machine of an ordinary type that the adjustment of the reading magnification (so called, to reduce/enlarge an image) in the sub-scan direction is controlled according to the scan speed of a scanner, when reading the image on an enlarged scale, the scan distance thereof is shortened in inverse proportion to the scan speed, with the result that the MTF in the above-mentioned C-C' direction is increased remarkably. That is, since the MTFs in the A-A' direction and C-C' direction are different greatly from each other, the quality of an image formed can be deteriorated in the specific direction of the image.
According to the above-mentioned Japanese Utility Model Publication No. Hei. 5-5719, although it is possible to prevent an image from being deteriorated in the image quality thereof in the specific direction of the image, if the photosensitive pixels thereof are arranged in point order, then the resolution thereof in the main scan direction is reduced down to a third. Also, since the photosensitive pixels are arranged in two lines which are shifted from each other, there is necessary a correction circuit in the sub-scan direction, which makes it impossible to have the merit of the point order arrangement that the structure can be simplified.
Further, in the above-mentioned Japanese Patent Application Publication No. Hei. 7-66948, not only because a high precision is required when manufacturing the diffraction gratings but also because a high mounting precision is required when they are mounted, there arise a problem that the manufacturing cost of the diffraction gratings is high and it is not easy to assemble the diffraction gratings. Also, when the diffraction gratings are interposed between the manuscript and lens, the cycles of the diffraction gratings must be set large but actually a sufficient cycle cannot be obtained due to its relationship with respect to the diameter of a luminous flux, with the result that the diffraction gratings are not able to provide satisfactory performance.